Fix crash of MultithreadingTest end2end tests. Extension platformANGLEContextVirtualization is currently only supported in the OpenGL backend, it's confused to validate it when the test is set up by WithNoVirtualContexts(). It'll generate crash when it's not OpenGL backend. Bug: angleproject:5636 Change-Id: Ib04fb4a3d29beaac5162e63af5ef1fd80c8a420c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2709821 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>